Various types of display devices, such as a cathode ray tube (CRT), a liquid crystal display (LCD), and a plasma display panel (PDP), which are to be used for a high-definition and large display television, have been developed.
The PDP includes phosphor layers for emitting three primary colors, red (R), green (G), and blue (B) so as to perform full color display by adding and mixing three primary colors (red, green, and blue). The PDP has a discharge cell, and generates visible light by exciting phosphor layers with ultraviolet rays generated by a discharge in the discharge cell, thereby displaying an image.
In an AC type PDP, an electrode for main discharge is generally covered with a dielectric layer, and performs memory driving to reduce a driving voltage. When the dielectric layer deteriorates due to an impact of ions generated by the discharge and hitting the layer, the driving voltage may increase. To prevent this increasing, a protective layer for protecting the dielectric layer is formed on a surface of the dielectric layer. For example, a protective layer made of material having high sputtering resistance, such as magnesium oxide (MgO), is disclosed in pp. 79-80 in “ALL ABOUT PLASMA DISPLAY” co-authored by Hiraki Uchiike and Shigeo Mikoshiba, published by Kogyo Chosakai Publishing Inc. in May 1, 1997.
The protective layer made of MgO provides the following problems. MgO generally tends to be charged in positive. Mg has positive divalency, a strong ionicity, and a high secondary electron emission coefficient (γ-coefficient), hence decreasing a discharge voltage of the PDP. However, MgO having such a high γ-coefficient has a lot of crystal defects, particularly oxygen defects, and may absorb H2O, CO2, and hydrocarbon gas in the defects. This may decrease initial emission of electrons, make the discharge unstable, raise the driving voltage, or increase a variation of temperature characteristics of the PDP. (For example, see “PLASMA DISPLAY” published by Kyoritsu Shuppan Co., LTD, pp. 48-49, or “VACUUM” vol. 43, No. 10, 2000. p. 973.)
Pure MgO, which is divalent oxide, has a strong ionicity and a lot of the oxygen defects, and thus tends to be charged positively. (For example, see “SOLID-STATE SCIENCE AND TECHNOLOGY” published by J. Electrochem. Soc. pp. 841-847, April, 1986.) Therefore, MgO may easily adsorb water or carbon dioxide gas. Just after being deposited in a vacuum chamber, MgO hardly absorbs impurity gas, such as water, carbon dioxide gas, or hydrocarbon gas. However, MgO may absorb the impurity gas during a period when the panel is taken out from the vacuum chamber to proceed a subsequent process, during the sealing of the panel, or during a later aging process. This is caused by oxygen defects produced in MgO crystal. Mg element provided on a boundary is coupled with hydroxyl group (OH) or CHX group in air through the oxygen defects and is stabilized.